The present invention relates to an endoscope system and, more particularly, to an endoscope system for observing an image of a subject on a monitor through an endoscope selected from a plurality of different types of endoscopes.
As is well known, endoscopes enable observation of the inside of a living body or the like, which cannot directly be seen by visual observation, and are widely used for diagnostic and therapeutic purposes mainly in the field of medical treatments. In recent years, video endoscopes have become widespread in which a subject image is converted into electric signals by a solid-state image pickup device, e.g. a CCD, so as to be observable on a monitor.
A variety of such endoscopes are used according to parts or regions to be observed. In use, these endoscopes are selectively connected to a light source unit, a signal processing device, etc. Accordingly, a plurality of different types of signal processing circuits must be prepared for various endoscopes. Under these circumstances, Japanese Patent Application Unexamined Publication (KOKAI) No. 6-304135, for example, discloses an apparatus for converting image signals from different solid-state image pickup devices into a video signal displayable on a monitor.
FIG. 22 is a schematic diagram of a conventional camera controller (image processor) 2. An image signal from an endoscope 1 connected to the camera controller 2 is converted into a digital signal by an A/D converter 8 and then sent to a signal converter 9. In the signal converter 9, the image signal, which corresponds to the type of solid-state image pickup device 5 mounted in the connected endoscope 1, is converted into a video signal displayable on a monitor. The video signal from the signal converter 9 is converted into an analog signal by a D/A converter 10, and an image is displayed on the monitor.
Japanese Patent Application Unexamined Publication (KOKAI) No. 8-313823 discloses an endoscope image processing apparatus that processes a video signal obtained from a video endoscope. This type of apparatus performs image processing corresponding to the spatial frequency characteristics of an image of a subject that are expressed by the product of the spatial frequency characteristics of the subject and the spatial frequency characteristics of the associated optical system. The endoscope image processing apparatus mainly performs image processing such as edge enhancement to aid in human visual recognition, thereby improving diagnostic capability.
PCT/US96/01514 discloses a technique of converting the spatial frequency characteristics of an optical system by using a phase filter to enlarge the depth of field of the optical system. When a phase filter is used, the spatial frequency response of the optical system lowers in the frequency range from a high-frequency region to an intermediate-frequency region, and a phase shift is produced at each frequency. To solve these problems, PCT/US96/01514 is arranged to execute signal processing for increasing the spatial frequency response of the optical system in the frequency range of from a high-frequency region to an intermediate-frequency region and for correcting the phase shift at each frequency, thereby obtaining an image of high quality with an enlarged depth of field of the optical system.
FIG. 23 is a diagram schematically showing the arrangement of a conventional imaging system arranged to enlarge the depth of field as stated above. The conventional imaging system includes an imaging optical system 6, a spatial frequency characteristic converter 7, a solid-state image pickup device 5, a spatial frequency restoring device 11', and a monitor 4. In this case, an individual system is needed for each of the required characteristics of the system, i.e. depth of field, focal length, and F-number.
When a technique of enlarging the depth of field of an optical system, for example, by using a spatial frequency characteristic converting device, e.g. a phase filter, is applied to an endoscope, it is necessary to use a spatial frequency characteristic restoring device for restoring the degraded spatial frequency characteristics of the optical system to obtain an image of high resolution.
In this regard, the existing endoscope systems have only the function of converting image signals from various endoscopes having different solid-state image pickup devices into a video signal displayable on a monitor.
In the existing endoscope image processing apparatus, image processing corresponding to the spatial frequency characteristics of each individual image is carried out. That is, image processing changes according to the kind of subject. However, it is difficult to obtain the spatial frequency characteristics of the optical system from the spatial frequency characteristics of the image.
Accordingly, the existing endoscope systems and endoscope image processing apparatus have the problem that it is impossible to accurately restore the spatial frequency characteristics of the endoscope optical system degraded by a spatial frequency characteristic converting device and to thereby obtain an image of high resolution.
Furthermore, the required characteristics of the endoscope optical system, e.g. focal length, depth of field, and numerical aperture, are specified variously according to each particular use application. Accordingly, there is a need for an endoscope system to which a plurality of endoscopes can be selectively connected regardless of whether or not an endoscope to be connected has a spatial frequency characteristic converting device and irrespective of the type of spatial frequency characteristic converting device, and which can produce an image of high resolution.
However, PCT/US96/01514 is a system in which a spatial frequency characteristic converting device and a spatial frequency characteristic restoring device are in one-to-one correspondence to each other. Therefore, the system cannot use an endoscope having a different spatial frequency characteristic converting device or an endoscope having no spatial frequency characteristic converting device.